In some situations there is a desire to provide a layer with a relief on a substrate. This may be the case in the provision of a frosted layer on clear glass or the provision of optical structures on printed matter to obtain visual effects. Although voluminous substrates are not excluded, it is envisaged that the layer having a relief is applied on a thin substrate, such as a card or a piece of paper. The substrate may be laminated but it may also be formed by a single layer. The substrate is preferably, but not necessarily formed by a security document such as an identity card, a personalized card in a passport or a credit card.
From the prior art a method is known, comprising the steps of securing the substrate with the surface onto which the relief is to be provided on top, applying a layer of a curable liquid on the upper surface of the substrate, bringing a mould having a counter relief into contact with at least a part of the upper surface of the liquid layer, curing the liquid layer while the mould is in contact with the curable liquid and separating the mould from the substrate.
This prior art method makes use of a rigid mould. This leads to the disadvantage that when the mould is brought into contact with the liquid on the substrate, air inclusions may develop, leading to bubbles in the liquid and to later to irregularities in the relief after curing. Further there the liquid may be pushed away from the substrate by the mould.
To avoid these disadvantages, another prior art method provides a method of the kind referred above wherein the mould is still rigid, but wherein the substrate is bent. Although possible if the substrate can be bended, this method has the disadvantage that, after making contact with the mould, the substrate must be brought back to its original flat shape before the curing can take place, requiring complicated machinery and likely leading to deformation of the relief. The substrate can be brought back to its original flat shape after curing, but this would lead to deformation and breakage of the relief layer.
The present invention aims to avoid these disadvantages, by providing such a method wherein initially the mould is brought into contact with the upper surface of the liquid layer in bent position while the substrate is kept flat and subsequently the mould is brought into contact with an increasing surface area, keeping the mould in a bent position until the complete liquid layer is in contact with the mould and at least the part of the mould in contact with the liquid layer being flat, keeping the substrate flat. The bent mould is kept in a form wherein the profile of the mould is a catenary. Upon contact of the mould with the surface, the profile of the mould is kept to be the same catenary. This is achieved by moving the ends of the mould which are guided by a manipulator not only in a vertical direction but also in a horizontal direction in such a way that the mould not being in contact with the surface of the liquid layer has the profile of the same catenary than before. By this guidance, the profile of the mould not being in contact with the surface can be kept as the same catenary over the whole process of bringing the mould into contact with the surface. At the end, the whole mould is in contact with the surface and after that there is no catenary profile but the mould is horizontal to the surface.
A catenary is the curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends. The curve has a U-like shape, superficially similar in appearance to a parabola, but it is not a parabola: it is a (scaled, rotated) graph of the hyperbolic cosine. Before the mould is in contact with the surface, it has a profile of a catenary. During the lowering process on the surface, this profile is kept by moving the ends of the mould along a curve that constrains the remaining profile of the mould not being in contact with the surface to that catenary of the free hanging mould with no contact with the surface. In practice, this might not be achieved mathematically precisely, but the method follows this theoretical curve approximately.
This method provides the gradual bringing into contact of the mould with the fluid layer on the substrate, avoiding any air inclusions. Further it avoids the urging of the liquid away from the substrate, as the process is gradual. Finally it avoids deformation of the relief layer either in liquid or in cured form with the associated problems.
If the substrate is a security document, the substrate is provided with a print, often with a personalized section. The relief layer needs to be transparent for visible light to allow the print of the substrate to be viewed. For further enhancement of the security it is preferred that the relief layer itself is also provided with a print. This extra print on the relief layer may cooperate with the print under the relief layer to give a visible effect, possibly in cooperation with the relief layer. The relief layer may have properties leading to a visible effect such as a lenticular effect. All these features enhance the security, that is, increase the difficulty of copying and falsification.
It is noted that in the above paragraph the mould is brought to the substrate; it will be clear that it is also possible to move the substrate to mould or to move both the substrate and the mould to each other. Any amendments to the apparatus required to perform these kinematic variations will be clear for a skilled man.
Further the present invention provides an apparatus for applying a layer having a relief on a flat surface of a substrate, the apparatus comprising a frame, securing means connected to the frame and adapted to secure the substrate with the surface of the substrate onto which the relief is to be applied, on top, a flat mould carrying a counter relief, a radiation source adapted to emit radiation curing the liquid applied in a layer on the substrate to a solid layer having a relief, wherein the mould is flexible and that the flexible mould has a profile of a catenary before being brought in contact with the upper surface of the liquid layer and the apparatus comprises a manipulator connected to the frame, the manipulator being adapted to initially bring the mould into contact with the upper surface of the liquid layer and to subsequently bring the mould into contact over an adjacent upper surface area of the liquid layer until the complete liquid layer is in contact with the mould wherein the mould is kept bent until the complete liquid layer is in contact with the mould and at least the part of the mould in contact with the liquid layer is flat, wherein the manipulator is adapted to guide the mould such that the profile of the mould not being in contact with the surface is kept as the same catenary over the whole process of bringing the mould into contact with the surface. With this guidance of the manipulators along a curve which is not only vertically moving downwards but at the same time moving horizontally outwards to a degree that the profile of the mould can be kept to follow the same catenary, constant conditions at the line of contact between the mould and the surface can be achieved.
For most mould materials, such as silicone, the flexibility of the mould allows bending of the mould over a single axis only. Assuming the axis of the bent of the mould to be parallel to the upper surface of the substrate, the mould is initially brought into contact with the substrate over a line.
However it is also possible that the mould is initially brought into contact with the substrate in a point. This requires the axis of the bend of the mould being tilted relative to the surface of the substrate during initial contact. Having such a tilted axis of bending prevents later contact of the mould over the complete surface of the substrate with the flat mould. To make contact over the full surface area of the substrate, the mould needs to be moved to a position with its axis of bending parallel to the surface of the substrate.
This embodiment also provides an apparatus wherein the clamps are connected to the yoke rotatably over an axis extending substantially between the centres of the clamps.